The present invention concerns a device to reduce vibrations in a hydraulic force transfer system. In particular, the invention relates to a device to suppress or reduce vibrations of the clutch pedal of a hydraulic clutch control in a motor vehicle.
The conventional hydraulic clutch control has a master cylinder which can be activated via a clutch pedal and which is connected to a reservoir. The master cylinder is hydraulically connected via a pressure line with the slave cylinder, so that, when the clutch pedal is pressed, the pressure generated in the master cylinder is transferred via the fluid column in the pressure line to the slave cylinder. As a result the clutch release bearing receives an activation force from the slave cylinder in order to separate via a release mechanism the clutch pressure plate from the clutch flywheel and hence the combustion engine from the gearbox of the motor vehicle.
In such hydraulic clutch controls, which can be regarded as quasi-stationary hydraulic force transfer systems in which there is no continuous flow of hydraulic fluid, the problem arises that vibrations from the combustion engine are transferred via the clutch plate, release mechanism, release bearing and fluid column in the pressure line between the slave cylinder and the master cylinder to the clutch pedal, so the clutch pedal vibrates perceptibly for the driver when he presses this down to release the clutch. The vibrations on the slave cylinder are transferred via the fluid column in the pressure line to the master cylinder as pressure pulses.
To solve this problem as illustrated by DE 195 40 753 C1 of the applicant it was proposed to connect an additional oscillator in the pressure line between the slave cylinder and the master cylinder. This additional oscillator has a housing, the base and/or cover of which is formed by a freely vibrating membrane whose edge is firmly clamped to the housing and which together with the housing delimits a chamber which can be connected via connections to the master cylinder and slave cylinder of the hydraulic clutch control so that the membrane is pressurized by the fluid column between the master cylinder and slave cylinder.
In this state of the art, because of a low frequency exciter vibration on the slave cylinder a low frequency pressure pulse is transmitted via the fluid column between the master cylinder and slave cylinder, where this pressure pulse excites the additional oscillator provided in the fluid column and tuned accordingly such that the additional oscillator vibrates at its inherent frequency which is higher than the frequency of the low frequency pressure pulse, so that the additional oscillator in the fluid column induces a higher frequency pressure pulse which the vibration-susceptible system comprising the master cylinder and clutch pedal cannot follow.
The structure or tuning of the additional oscillator known from DE 195 40 753 C1 has amongst others the advantage that to reduce the vibration, less volume increase is required than in solutions based on another action principle, such as for example in GB-A-1 562 709 or U.S. Pat. No. 5,070,983 in which a membrane, which together with a housing delimits a pressure chamber communicating with the fluid column, merely follows the pressure pulses in the fluid column in order to damp these. Together with the relatively larger volume increase this method has higher undesirable declutching losses, which on activation of the clutch pedal are perceptible for the driver in particular as a softer clutch control, a greater travel on the clutch pedal and the lack of a pressure point.
Due to the reduction in volume increase caused by the active principle according to the state of the art shown by DE 195 40 753 C1, the stop surfaces for the membrane on the side of the membrane facing away from the pressure chamber can be omitted; according to the disclosure of U.S. Pat. No. 5,070,983, these are provided for example in the form of a projection or plate spring in order to prevent excess curving of the pressurized membrane.
It has in practice however been found that the additional oscillator according to DE 195 40 753 C1 could be improved for certain applications in respect of response behavior and volume increase. The present invention is therefore based on the task, starting from the state of the art according to DE 195 40 753 C1, of producing a simply designed device to reduce vibrations in a hydraulic force transfer system which can easily be modified in respect of response behavior and volume increase to the requirements in each case.
According to the present invention, there is provided a device to reduce vibrations in a hydraulic force transfer system which has a master cylinder and a slave cylinder connected to the master cylinder via a fluid column, with a housing and a membrane, the membrane having an edge by which it is attached to the housing, wherein the housing and the membrane delimit a chamber which can be connected via a connection to the master cylinder and via a further connection to the slave cylinder, so that the membrane can be pressurized hydraulically by the fluid column between the master cylinder and slave cylinder, and wherein the membrane, at least in a rest state of the hydraulic force transfer system, is pressed by means of a spring element against the direction of the hydraulic pressurization of the membrane against a support surface on the chamber side.
As a result of this defined pressure from the spring and associated more difficult deflection of the membrane, with otherwise the same geometry and same material of membrane, the volume increase of the device can be reduced in comparison with the known state of the art, or by suitable selection or design of the spring element can easily be adapted to the requirements of the given system conditions in the manner of a modular system. Tests by the applicant have here led to the surprising result that with the structure of the device according to the invention nonetheless a vibration reduction occurs which is comparable to the vibration reduction achieved with the freely oscillating membrane according to DE 195 40 753 C1. It is assumed that the active principle of the device according to the invention here corresponds to the known state of the art, where, depending on the selection or design of membrane and spring element and the amplitude of the pressure pulses occurring in the fluid column, the system consisting of membrane and spring element or just the membrane oscillates in its xe2x80x9cfreexe2x80x9d areas i.e. the areas which are not directly pressurized by the spring element or do not lie directly between the spring element and the supporting surface on the chamber side.
Furthermore the spring element and supporting surface on the chamber side constitute a mechanical support for the membrane which allows the use of membranes of reduced thickness and/or greater diameter without the risk of mechanical overload of the membrane. The possible enlargement of the ratio of diameter to thickness of the membrane can advantageously lead to a reduction in hysteresis of the device i.e. its pressure loss, and the elasticity of the device, i.e. the ratio of pressure loss to volume increase, or its adaptation to the relevant requirements as will be described below, whereby the device can react more sensitively to pressure pulses in the fluid column and have an improved response behavior and overall a better vibration-reducing effect.
The housing preferably has a projection extending centrally from the base of the chamber in the direction of the membrane, which in a simple manner forms the supporting surface for the membrane.
The housing suitably has an annular groove concentrically surrounding the chamber to receive a sealing element lying against the membrane which seals the chamber against the environment. Preferably, the housing has a ring-cylindrical section between the chamber and annular groove against which the membrane lies at least in the rest state of the hydraulic force transfer system, where the ring-cylindrical section has at least one connecting channel which connects the chamber with the annular groove so that on assembly of the device in the hydraulic force transfer system secure filling of the chamber with hydraulic fluid is possible without air inclusions remaining in the annular groove.
A face of the ring-cylindrical section and the support surface on the projection can lie in one plane so that the membrane loaded by the spring element in the rest state of the hydraulic force transfer system is advantageously held flat. Depending on the requirements concerned it is however also possible to provide an offset between the supporting surface on the projection and the face surface on the ring-cylindrical section in the axial direction of the device and/or to chamfer or make convex the supporting surface at the projection and/or the face surface at the ring-cylindrical section so that the membrane is held curved convex or concave under a defined pretension.
A cover can be provided on the essentially pot-like housing which together with the housing clamps the membrane at its edge whereby the spring element rests with its end facing away from the membrane on the cover. Such a cover can not only form the support required for the spring element in a simple and stable manner but also ensure that the hydraulic force transfer system in which the device is fitted cannot leak if the membrane is damaged and thus cannot fail.
In principle it is possible to attach the cover to the housing by means of a bayonet closure, for example, or by welding the housing and cover together. However, a design is preferred according to which the cover is attached to the housing by means of a threaded connection where the cover has an external threaded section which can be screwed into an internal threaded section of the housing. Such a threaded connection firstly allows easy replacement of the membrane. Secondly by means of a threaded connection the retention or clamping forces for the membrane can be set positively.
The threaded connection is preferably structured as both a sawtooth and a round thread such that at least the internal threaded section of the housing at the external diameter of the thread profile is rounded in a round thread-like manner and at the internal diameter of the thread profile has a rounded sawtooth thread-like form. If the housing and cover are injection-molded of plastic, such a threaded connection is easily structured at the same time. In addition the threaded connection described above has advantages in that due to the sawtooth thread-like structure on the internal diameter of the thread profile on the housing a greater thread bearing depth can be achieved than with a conventional trapezoid thread, whereas the round thread-like structure on the external diameter of the thread profile on the housing ensures a reduction in notch effect in comparison with conventional trapezoid thread, which is particularly important for the durability of the housing when plastic is used as a housing material.
The cover preferably has a central receiving and guiding section for the spring element which easily prevents the spring element from moving radially in the device and thus unintentionally pressurizing the membrane at different points.
In principle the spring element can be a spring ring or plate spring. However the use of a coil compression spring is preferred.
The spring element can be surrounded concentrically by a flange sleeve which has a ring collar arranged between the end of the spring element facing the membrane and the membrane. This simply prevents any sharp-edged parts of the spring element from damaging the membrane. The face of the ring collar facing the membrane can for example be crowned. In particular for production reasons, however, a flat structure of this face is preferred.
The flange sleeve can be suitably guided in an axial mobile manner in a receiving and guiding section of the cover. Preferably, the flange sleeve has here a peripheral projection on the external periphery on the end facing away from the membrane whereas the receiving and guiding section of the cover has a peripheral projection on the inner periphery on its end facing the membrane, where the peripheral projection of the flange sleeve has an external diameter slightly larger than the internal diameter of the peripheral projection on the receiving and guiding section of the cover. This design of cover and flange sleeve firstly simplifies assembly of the device because the flange sleeve, after receiving the spring element, can be locked with the receiving and guiding section in the receiving and guiding section of the cover so that when the cover is screwed in/attached to the housing, there is no multiplicity of components to be handled. Secondly the peripheral projections on the flange sleeve and receiving and guiding section of the cover ensure defined and free-moving guidance of the flange sleeve in the receiving and guiding section of the cover.
Suitably the membrane can consist of spring steel. With a spring steel membrane with a ratio of diameter to thickness of the membrane preferably between 40 and 160, more preferably between 60 and 90, with the device according to the invention particularly good results can be achieved in the reduction of vibrations in a hydraulic clutch control.
The housing and/or cover and/or flange sleeve are preferably injection-molded of plastic. In the known state of the art as a material for the housing of the additional oscillator an aluminum alloy was used where the semi-finished product for the housing was extrusion-molded then trimmed and finally machined. With injection molding of plastic, however, the housing can advantageously be produced in one working process straight from the tool i.e. with no further processing steps. It is evident that by this material substitution productivity can be considerably increased and the production costs for the housing essentially reduced.
The device according to the invention can be connected in a hydraulic force transfer system finally in the fluid column between the slave cylinder and master cylinder such that with its corresponding connection it is directly plug-connected to the master cylinder or alternatively its housing is an integral part of the housing of the master cylinder. In the trials of the device according to the invention, it was found that the vibration-reducing effect of the device according to the invention, stiffer than the known state of the art due to the spring pressure on the membrane, is the better the closer it is arranged to the master cylinder.